Due to the fact that shift elements, designed as a clutches or brakes, of commonly known automatic transmissions are constructed to transfer very high torques, coasting downshifts show a substantial sensitivity to defects regarding the generation of torque deliverable by a clutch to be released or engaged. This sensitivity to defects causes a degradation of desired shift comfort when the optimum shift procedure is not maintained.
Coasting downshifts which are not initiated by a driver-activated request such as activation of a selector lever or selector switch, but are initiated by way of a shift strategy programmed in a transmission control unit and which lead to irregularities in the profile of an output torque and are perceived by the driver as unpleasant.
The above problem is further intensified by the increased inertia of the rotating masses in the transmission, as well as by the inertia of the rotating masses of other components of a vehicular drive train that are involved in the shifting process, which must be accelerated or decelerated during the changing of a gear ratio. This results from the fact that during coasting downshifts, the rotating masses which, in relation to the motor of a drive train positioned ahead of the clutch, must be driven to the rotational speed level of the target gear or the gear ratio that is to be set in the transmission device by the clutch to be engaged.
This process causes a dip in the acceleration which depends on the amount of inertia, as well as on the rotational speed gradient. The level of such an acceleration dip can be reduced by way of longer shift periods, which increases the possibility, that during such a shifting procedure, the driver will initiate a performance command and/or activate the accelerator pedal. A driver-initiated performance command causes a spontaneous alteration in the desired operating condition of a vehicle drive train, whereby this operating condition must be achieved within a short operating period with the spontaneity that the driver desires.
However, this procedure has the disadvantage of increasing control and adjustment complexity which, in turn, increases the development and manufacturing costs of a transmission.
In order to reduce and/or eliminate the perceived acceleration dip at the output during coasting downshifts, changes have been implemented to support the acceleration of the inertia within the drive train during the coasting downshift by way of positive engine control.
Implementation of the positive engine control during the above-described operating profile of drive trains and/or automatic transmissions is potentially problematic, in addition the so-called revving of a combustion engine is usually audible to the driver and during unfavorable temporal sequences that can be caused by program execution times, can also negatively affect the operation of a combustion engine.
Further from practice, we know of automatic transmissions which are designed with a free wheel so as to improve shifting quality during coasting downshifts, the free wheel opening at an output rotational speed which is less than that of the driving mechanism. Such free wheels require additional space, however, and increase the manufacturing costs of a transmission unit.
It is the purpose of the present invention to provide a comfortable method of shifting during coasting downshifts in automatic transmissions without additional structural aids such as a free wheel.